This invention relates to a method of stressing an annular seal member, a method of sealing an annular gap, and a sealing arrangement particularly, although not exclusively, applicable to plug valves.
Plug valves are commonly provided with annular seat members surrounding respective ports in the valve body and engaged between the inner surface of the body and the surface of the plug. The seat members serve to provide seals between the plug and body, and, in the case of floating plug valves, to provide bearing surfaces for the plug.
In an attempt to maintain positive sealing engagement between the seat members and the surfaces of the plug and body when under low service pressure, the valve can be arranged so that each seat member is resiliently stressed to react against those surfaces. In this way, changes in relative dimensions resulting from temperature fluctuations, variations within manufacturing tolerances and movement of the plug under varying service load conditions can be accommodated by resilient deformation of the seat members.
A difficulty arises, however, in the use of material for the seat members which has poor elastic properties and a tendency to deform plastically, for example, fluorocarbon plastics material. Fluorocarbon plastics material, especially PTFE, is particularly valuable as a seat member material because it retains most of its useful properties over a wide range of temperatures, is unaffected by nearly all known chemical substances and has excellent anti-friction properties. One property of the material, however, is that when subject to stress over a period of time the material tends to deform plastically to reduce the stress, the effect being particularly pronounced at elevated temperatures. Moreover, the instantaneous elastic range of PTFE is rather limited. That is to say, the percentage deformation to which the material can be subjected so that it will return immediately to its original shape or size upon removal of the deflecting influence is small (compared, for example, with that in the case of rubber).
In a ball plug valve, the loading to which the seat member is subject with the valve closed under service pressure can be many orders of magnitude greater than the assembly stress (that is to say, the resilient stress "built-in" to maintain positive sealing engagement, as indicated above, under low service pressure). Such loading can result in severe local plastic deformation of a PTFE seat member, loss of the assembly stress and consequential leakage at low pressure. Similar plastic deformation can occur as a result of large temperature fluctuations. The ratio of the thermal expansion co-efficients of steel (of which the body and plug are commonly made) and PTFE is of the order of 1:10, so that at elevated temperatures, the seat member expands relatively to the other components. As a result, the seat member can be subject to substantial compressive hoop stress from the bore of the seat member recess in the valve body, and radial compressive stress owing to confinement between the plug and body surfaces. At low temperatures the seat member contracts so as to grip tightly around the ball plug, or jam between the plug surface and the shoulder of the seat recess. This results in deformation of the seat member, increased operating torque and increased wear.
Similar problems arise in the case of shaft and piston seals. In a shaft or piston sealing arrangement, it is necessary for positive sealing to maintain a minimum loading of the annular seal member against the surface of the shaft or piston on the inner side of the ring and the surface of the shaft housing or cylinder chamber on the outer side of the ring. On the other hand the loading should not be too great as this increases friction and wear.
A common sealing arrangement comprises a sealing ring, for example an `O-ring`, compressed in the annular gap between the surface of the shaft or piston and the surface of the shaft housing or piston chamber. The ring is thus subject to radial compression between those surfaces. A difficulty with this arrangement, however, is that the radial stress in the sealing ring varies rapidly with variation of the thickness of the annular gap. Thus slight eccentricity of the shaft, or slight variation in the diameter of the piston chamber, can cause substantial variation in the loading on the sealing ring resulting in either leakage or excessive wear of the ring. Moreover, the life of the sealing ring in such an arrangement is relatively short since even in ideal conditions, only slight wear of the ring produces a significant reduction in the sealing load exerted. This can only be compensated by increasing the initial compression of the ring or making the ring of softer material and in both cases this serves to accelerate wear.
The above problems are particularly pronounced in the case of sealing rings of fluorocarbon plastics material, for example, PTFE.